1. Field of the Invention
This invention relates generally to turbine blades as for use in a gas turbine engine and, more specifically, to a method of, and apparatus for, assuring that all blades on a rotational disk are balanced and have desired airflow characteristics within predetermined tolerances.
2. Description of the Prior Art
In a turbine assembly such as an aircraft engine or the like, a plurality of turbine blades are connected with and extend radially from a rotational disk or hub. Each blade includes a root form portion which is serrated to fit within similarly configured slots in the disk to lock the blades in place and may have a shroud for holding the blade at its tip end. Each turbine blade has a precise configuration, the tolerances of which are determined by the accuracy of the root form and of the shroud, if it has one.
The present invention relates to a system for securely holding a turbine blade during machining or grinding of the shroud and of the serrations in the root form. In accordance with the invention, the root form and the shroud may be ground more accurately, thereby improving the tolerances of the entire configuration of the blade.
After a turbine blade has been cast, it was known in the art to arrange the airfoil portion of the blade in a mold or matrix block and to fill the mold with molten lead which hardens to form a lead matrix block. Typically, the root form portion of the blade extends from the lead matrix block, the block is mounted in a fixture, and the serrations are ground in the protruding root form portion of the blade.
A major drawback of the use of lead matrix blocks is that the lead is porous and flexible, even when in the solid state. Thus, the lead matrix block cannot be securely held during grinding of the root form, whereby inaccuracies in the root form are developed. Since the root form provides the basis for all of the critical dimensions in the blade, the entire blade will be inaccurate when the root form is not ground to the proper specifications.
Furthermore, the prior methods of manufacture are relatively expensive and time consuming. A major expense is the cost of the lead used to form the lead matrix block for each turbine blade. Additional tooling including the lead mold, the molten lead supply, and the fixture for holding the lead form are also required. Finally, a great deal of time is wasted while waiting for the lead to solidify in the lead matrix form and in breaking the lead away from the blade following the grinding operation.
Turbine blade holding devices are also known in the patented prior art as evidenced by the U.S. Pat. Nos. 3,331,166 to Brenning, 3,331,166 and 4,400,915 to Arrigoni. While these devices normally operate satisfactorily, they are designed for machining the air foil portion of the blade rather than the root form portion.
A significant problem with the prior art resides in its failure to account for variations in the shape and size of an airfoil resulting from the casting operation. No two blades are absolutely identical no matter how carefully the casting process is performed. For that reason, positioning of the blade within a preestablished, but minimized, range of tolerances is important before performing the operation of grinding either the root form or the shroud. A major drawback of known blade holders is that they are incapable of accurately and securely holding a plurality of successive turbine blades during grinding of the root form portions of the blades, whereby each root form, and thus each blade, will be uniform in dimension and configuration.
A further improvement in blade holding devices is disclosed in the U.S. Pat. No. 4,638,602 to Cavalieri. This recent patent discloses a number of arrangements intended to hold a turbine blade relative to its stacking axis during grinding of its root form portion. While Cavalieri mentions a number of designs adapted particularly for use with blades whose thickness varies, they differ considerably with no preferred construction being presented. In some instances, the extreme tip of the airfoil, which is that region most subject to major dimensional variations from one casting to the next, is caused to rest on the datum plate. This positioning of the airfoil would cause precisely the type of problem which the present invention is intended to prevent.